The present invention relates to a fuel cell system, and more particularly to a technology for controlling a fuel cell system that can reliably interrupt the treatment of activating a catalyst layer of a fuel cell, while avoiding the overcharging of a power storage device.
In a fuel cell system, the output voltage of a fuel cell decreases as oxygen is adsorbed by a catalyst layer of the fuel cell during operation of the system. In such cases, a treatment has conventionally been performed by which the supply of oxygen to the fuel cell is temporarily stopped and the output voltage of the fuel cell is decreased to a reduction region of the catalyst layer, thereby activating the catalyst layer of the fuel cell stack (that is, a reduction treatment is performed).
As the initial step of such activation treatment, the supply of an oxidizing gas (for example, air) is stopped and then control is performed to reduce the output voltage of the fuel cell to a target reduction voltage value.
Further, the control is so performed that when it is necessary to terminate the activation process in the course of decreasing the output voltage of the fuel cell to the target reduction voltage value, the supply of the oxidizing gas (for example, air) is immediately restarted. Such a necessity to terminate the activation treatment can occur, for example, when a cross leak occurs in an electrolyte membrane of the fuel cell and the fuel gas (for example, hydrogen) leaks from an anode electrode (electrode to which the fuel gas is supplied) to a cathode electrode (electrode to which the oxidizing gas is supplied). Thus, because the electrolyte of a fuel cell is composed of a porous material to increase the surface area of contact of both electrodes with the oxidizing gas or fuel gas, it is possible that a cross leak will occur. The upper limit of the output voltage of the fuel cell is limited by a voltage conversion unit (for example, a converter) that can set the upper limit of the output voltage of the fuel cell.
According to the technology relating to the catalyst activation treatment of fuel cells disclosed in Japanese Patent Application Laid-open No. 2005-346979, a low-voltage battery is used as a reserve device constituting a hybrid fuel cell, and an extra power that increases following the decrease in voltage during catalyst activation treatment of the fuel cell is charged into the battery, thereby effectively using the extra power.
Japanese Patent Application Laid-open No. 2003-115318 discloses a technology of inducing a reduction reaction in oxygen by reducing the cell voltage to 0.6 V or less and passing a large electric current, thereby activating a platinum catalyst layer, as another technology relating to such catalyst activation treatment.
However, with the above-described conventional technology, when the activation treatment is interrupted in the course of decreasing the output voltage of the fuel cell to the target reduction voltage in order to activate the catalyst layer of the fuel cell, the control is performed, as described above, so as to restart immediately the supply of the oxidizing gas. Therefore, when the supply of the oxidizing gas is restarted, the output voltage of the fuel cell drops below the standby voltage. Where the supply of the oxidizing gas is restarted in a state in which the output voltage of the fuel cell is lower than the standby voltage, the output power of the fuel cell rapidly increases, sometimes causing overcharging of a power storage device such as a secondary battery.
Here, a case in which a cross leak occurs in an electrolyte membrane of a fuel cell and a fuel gas (for example, hydrogen) leaks from an anode (fuel gas) electrode side to a cathode (oxidizing gas) electrode side will be considered as a case in which the activation treatment is interrupted. In such case, a large amount of an oxidizing gas (for example, air) has to be supplied to the cathode electrode in order to decrease the concentration of fuel gas (for example, hydrogen) in the exhaust gas. However, where a large amount of an oxidizing gas (for example, air) is supplied to the cathode electrode in the course of catalyst activation treatment (referred to hereinbelow as “refreshing”), the power generated by the fuel cell rapidly increases and extra power that can be charged into a power storage device is generated. In particular, in the course of refreshing, because a load device is stopped and, therefore, power consumption is low, the power storage device is overcharged.